Limited supply of phosphorus (P) reserves and the increasing demand for food production have created a strong demand for P fertilizers. Global P depletion is one of the important challenges in the 21st century. However, runoff from fields and feedlots introduces large quantities of P-containing fertilizers and animal wastes into surface waters, causing water pollution and eutrophication. Such runoff is a danger for the denizen of water and the whole ecosystem on a broader prospective. Eutrophication caused by municipal and industrial wastewaters was reported even at low concentrations of P (less than 1 mg/L). In order to control algal growth, the U.S. EPA water quality criteria stated that phosphate should not exceed 0.05 mg/L for streams discharging into lakes or reservoirs, 0.025 mg/L within a lake or reservoir, and 0.1 mg/L for streams or flowing waters not discharging into lakes or reservoirs. Improved management strategies and treatment technologies are highly desired in order to reduce agricultural runoff and to capture and recycle P before it reaches waterbodies.
Many approaches have been developed to remove dissolved phosphate from wastewaters prior to their discharge into natural water bodies and runoff, including physical, chemical, and biological treatment methods. Typically, phosphate is separated from wastewaters by adding Al-, Fe-, or Ca-based coagulants and allowing the precipitates to settle out. A common drawback of this coagulation process is the high costs associated with the use of metal salts and the treatment of the remaining sludge.
The enhanced biological phosphorus removal (EBPR) process utilizes polyphosphate-accumulating organisms (PAO) to take up and polymerize inorganic phosphate to produce polyphosphate (polyP). P level of lower than 0.11 mg/L can be achieved in the effluents after the EBPR treatment of municipal wastewater. However, the performance of EBPR can be dramatically reduced due to many environmental and operating factors, making this process unstable. In addition, the inability to isolate the responsible microorganisms in EBPR and to verify their biochemical metabolism appeared to limit the development of a better understanding of the operating metabolic pathways and the characterization of the entire microbial ecology of the systems, thus hampering further improvement of the EBPR system.
Adsorption has attracted increasing interests for phosphate removal from wastewater due to the easiness of design and operation and no additional production of sludge. This method has also been considered as an effective approach for recycling P from wastewater effluents. Over the past decade, various adsorbents have been developed for phosphate removal from wastewaters including agricultural waste and by-products, anion-exchange resins, iron-oxide based adsorbents, aluminum-containing materials, and layered double hydroxides. However, additional filtration or centrifugation steps are likely needed for the separation of sorbents from aqueous solutions.
Magnetic nanomaterial based sorbents are very attractive due to their high surface area and facile solid-liquid separation under an applied magnetic field. Their surface areas per unit volume can be on the order of 5×107 m2/m3 for a 10% dispersion of 15-nm particles. Adsorption of phosphate onto amine functionalized magnetic nanoparticles through electrostatic attraction has been reported. However, the simple electrostatic adsorption might suffer from the interference of co-existing anions in wastewater. Core shell materials with Fe3O4 as the core (˜600 nm diameter) and ZrO2 as shell (˜10 nm thickness) have also been used to remove phosphate, with adsorption capacities ranging from 8 to 39 mg P/g.